In a 2 to 1 decision yesterday, the U.S. Court of Appeals for the District of Columbia Circuit struck down an exemption that for nearly 15 years has allowed refineries, chemical plants and other industrial facilities to exceed federal air pollution limits during certain periods of operation.

Environmental groups hailed the ruling, which overturned a provision, enacted under President Bill Clinton, that permits industrial operations that are starting up, shutting down or malfunctioning to emit more toxins into the air than is normally allowed. The Environmental Protection Agency and an array of business groups argued that the exemption was essential, but the court determined that it was illegal.

The ruling affects sources of air pollution across the country: Texas alone has 250 industrial sites, including oil refineries, chemical plants and petrochemical plants, that are affected.

"Citizens will be able to breathe cleaner air with greatly reduced levels of toxic chemicals released, especially people living in the fence-line neighborhoods near Texas's refineries and chemical plants where start-up, shutdown and maintenance emissions have been a huge pollution problem for decades," said Neil Carman, a former Texas state refinery inspector who is now clean air director for the Sierra Club's Texas chapter.

It is unclear whether the EPA will appeal. Agency spokesman Jonathan Shradar said, "EPA is disappointed by the court's ruling, and we will be reviewing the decision as we determine steps moving forward."

The agency created the exemption in 1994, and Bush administration officials broadened the interpretation of the provision over time. This made it subject to judicial review, and a coalition of advocacy groups including the Environmental Integrity Project, the Sierra Club, the Louisiana Environmental Action Network, the Coalition for a Safe Environment and Friends of Hudson challenged the provision's legality in court.

"What they did is take a bad provision and turn it into an almost complete barrier to enforcement," said Earthjustice attorney Jim Pew, who argued the case on behalf of the coalition. "This was an attempt to make all of the air-toxics laws unenforceable, and they almost got away with it."

Pew said industrial facilities routinely violated federal air standards under the guise of malfunctions, but industry representatives said these excesses were a necessary part of operations. Richard Alonso, a lawyer at Bracewell & Giuliani who represents oil refineries, said the plants often need to flare off volatile gases when they start up.

"These sources are already doing the best they can to reduce their emissions during these times," said Alonso, who worked in the EPA's office of enforcement and compliance from 2001 to 2007. "It's a big setback for refineries, and not just refineries but other industry sectors."

Alonso predicted that the next administration would have to devote "big resources" to rewriting regulations to comply with the court's ruling.